Patients who present nutritional risk upon hospital admission are more likely to have worse clinical outcomes. Evaluating muscle thickness with ultrasound is a predictor of muscle mass loss in pediatric patients. We reviewed the muscle mass loss detection through ultrasound to assess the body composition of pediatric patients. We found an association between muscle reduction, as detected by ultrasound, and the duration of mechanical ventilation, nutritional deficits in energy and protein intake, and the age-related skeletal muscle atrophy of the limbs. All studies reported a reduction in muscle thickness of more than 10% during hospitalization. There is a lack of standardization in muscle mass assessment protocols and established cut-off points in critically ill hospitalized children. Further studies are needed to establish an accurate and standardized analysis for monitoring muscle changes using ultrasound.